The crystal structure of the catalytic domain of tyrosine hydroxylase (TyrOH) to 2.3 E resolution has recently been completed in our laboratory by the MIRAS method. Structural studies are in progress of TyrOH with substrates, inhibitors, and the required biopterin cofactor bound. Using molecular replacement with TyrOH, structural studies have begun for the closely related enzyme phenylalanine hydroxylase. These high resolution structures will provide new knowledge about the function of this family of enzymes as well as the role of single amino acid mutations that cause human diseases such as L-DOPA responsive parkinsonism and phenylketonuria (PKU).